I know it'll sound like I'm just coming at the same thing from the other side, but the purpose of the mass damper was not to allow for more compliant suspension, but to stabilize the aero platform.
And the way the regs were written at the time, "any specific part ofthe car influencing its aerodynamic performance must be rigidly secured to the entirely sprung part of the car (rigidly secured means not having any degree of freedom)," and "must remain immobile in relation to the sprung part of the car".
The regulators fully understood its benefit–as did McLaren, Ferrari, Toro Rosso, Honda, and Midland, who were all testing their own versions of that system–but it's hard to see how a moving weight strapped to the sprung part of the car in order to improve the aero platform is anything but illegal.
That was precisely the misunderstanding. The purpose of the device was to improve mechanical grip by allowing the tires to have a more uniform pressure with the ground, removing some of the influence of impulses applied to the tire/suspension and the attendant lateral oscillations that result.
With such a rigid interpretation of the regulations, one could say the anti roll bar, spring, or damper would also be in violation of the regulations, as they have knock on effects concerning the aerodynamics.
It improved both mechanical and aero grip at the same time, but the primary purpose, as per Toyota's senior chassis engineer, was to be able to stabilize the aero platform, specifically around pitch. Which makes sense: same reason active suspension came into being, even at the expensive of grip. Aero gains are more productive, on balance, than mechanical grip gains, at least in Formula 1.
It was moveable ballast, plain and simple. In its early guise, was totally unconnected from the suspension. So that's a pretty easy distinction to make vs. springs/dampers/anti-roll bars.
And the way the regs were written at the time, "any specific part ofthe car influencing its aerodynamic performance must be rigidly secured to the entirely sprung part of the car (rigidly secured means not having any degree of freedom)," and "must remain immobile in relation to the sprung part of the car".
The regulators fully understood its benefit–as did McLaren, Ferrari, Toro Rosso, Honda, and Midland, who were all testing their own versions of that system–but it's hard to see how a moving weight strapped to the sprung part of the car in order to improve the aero platform is anything but illegal.